1. Field of the Invention
The invention relates to pressure sensors. In particular, it relates to high pressure sensors having increased reliability, thermal stability and adhesion properties, while being inexpensive to fabricate.
2. Description of the Related Art
For years, those skilled in the art have made continuous efforts to develop pressure sensors that are low in cost and capable of being mass produced, while exhibiting high reliability, sensitivity and linearity. Certain-conventionally known pressure sensors have been known to include semiconductor materials with a micromachined sensing diaphragm. In the processing of such structures, a thin diaphragm is often formed in a silicon wafer through chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical conductivity changes with strain, such that deflection or deformation of the diaphragm causes a change in resistance value of the piezoresistive element. These changes correspond to the magnitude of pressure applied to the diaphragm. However, silicon is susceptible to chemical attack and erosion, particularly in environments such as where a high-pressure medium is to be sensed, such as automotive applications that involve sensing pressures of brake fluid, oil, coolant, transmission fluid, hydraulic fluid, fuel, or steering fluid, at pressures of two atmospheres or more. For such applications, a pressure sensor must be physically resilient, and resistant to the hostile environment of a sensed medium.
Presently used methods for producing media-compatible sensors include enclosing a silicon sensing chip in an inert fluid, such as a silicone oil or gel, and then further separating the sensing chip from a medium to be sensed with a metal diaphragm, such that pressure must be transmitted through the metal diaphragm and the fluid to the sensing chip. Unfortunately, the manufacturing processes for these sensors are expensive and complex. As a result, these sensors are unsuitable for mass production.
Another known process involves the formation of a capacitor plate on a ceramic diaphragm, which is then bonded to a base with a second capacitor plate. The use of a chemically-resistant and mechanically-tough ceramic materials, such as aluminum oxide or zirconium oxide, allow the diaphragm to directly contact the medium whose pressure is to be measured. As the ceramic diaphragm deforms or deflects under the influence of pressure, the space between the capacitor plates changes, thereby causing a change in capacitance that corresponds to the applied pressure. However, the circuits required to detect such capacitance changes are often complex and subject to noise corruption. Furthermore, attaining an adequate seal between the ceramic diaphragm and the base for high pressure applications is often difficult.
Yet another known approach involves the use of a chemically resistant ceramic diaphragm having thick-film piezoresistors screen-printed thereon. As with ceramic capacitive pressure sensors, the ceramic material allows direct contact with the medium whose pressure is to be sensed, eliminating the need for protective packaging. However, while the signal detection circuitry may be less complicated, it is difficult to reliably seal the ceramic diaphragm with a base.
Further, it is known in the art to employ a metal diaphragm as the sensing element. Because metal diaphragms generally deform more for a given thickness and pressure than ceramic diaphragms, which exhibit lower elongations before breaking and are therefore designed to deform less under pressure, sensing is performed by thin-film polysilicon resistors. The metal diaphragm is first coated with a thin-film layer of typically silicon dioxide or silicon nitride to electrically insulate the diaphragm from thin-film resistors and their conductors. A thin-film polysilicon layer is then deposited on the insulating layer by chemical vapor deposition (CVD), and then etched through a mask formed by spinning a liquid photoresistive material onto the polysilicon layer. The photoresistive material is exposed and developed to obtain the pattern required for the resistors, and thin-film conductors are then formed by evaporation. However, the equipment necessary to deposit the insulating and polysilicon layers is expensive, and deposition rates are extremely slow. Deposition of the thin-film layers requires multiple patterning, exposure, developing and stripping steps for the required thin-film spun-on photoresists and metallization, and must be carried out in a controlled environment to avoid surface defects. In addition, such processes typically deposit thin-films no thicker than about 10,000 angstroms, requiring the surface of the metal diaphragm to be extremely smooth to avoid defects.
Metal diaphragm sensors have also been produced with thick-film metal oxide resistors. A thick-film oxide layer is formed by printing and firing a thick-film ink, after which a thick-film metal oxide layer is printed and fired on the insulating layer to form resistors. Thick-film conductors are typically employed with this type of sensor. However, since thick film materials fire at very high temperatures, for example in the range of 800° C. to 1000° C. and above, the diaphragm must then be formed of a metal having excellent high temperature corrosion resistance. While several high temperature resistant specialty metals are known in the art, they are very expensive and difficult to machine. Another option is to attach a small metal diaphragm “button” of specialty metal to a pressure port base made of a cheaper material. However, these structures are often unreliable at higher pressures.
Clearly, a need for further improvement exists in the art of pressure sensors, particularly in the formation of pressure sensors which are lower in cost to produce, while still being reliable at high pressures. The present invention provides a solution to this problem. The invention provides a pressure sensing apparatus which includes a thin disc of a specialty metal or ceramic, having a ceramic material layer and piezoresistive elements formed thereon. This structure is bonded to a diaphragm assembly on a pressure port base constructed of a low cost metal. The bonding process is performed at low temperatures, (<700° C.), so that the diaphragm assembly and pressure port do not require high temperature corrosion resistance, and can thus be formed of less expensive materials.
The inventive pressure sensing apparatus provides a lower cost alternative to conventional high pressure sensors since less material is used, less expensive materials are used, and fabrication is less complex. The inventive apparatus is also more reliable and exhibits greater thermal stability than conventional high pressure sensors.